1. Field of the Invention
This invention relates to an apparatus for cooling molten plastic extrudate as it leaves the extruder in order to hold the size and shape of the extrudate as the molten plastic cools and hardens. The vacuum sizing apparatus has a vacuum tank containing water through which the extrudate passes as it cools and hardens.
2. Description of the Prior Art
Many efforts have been made in the plastics industry to provide for the controlled cooling and accurate sizing of molten plastic extrudate after it leaves the plastic extruder. The extrudate, which is often cylindrical pipe or extruded hollow shapes can easily be deformed if excessive pressure is applied to the outer surface of the hollow shape without equalization of the pressure on the internal passages of the shape.
To overcome the deformation of plastic extrudate as a result of pressure on the external surfaces, vacuum cooling and sizing tanks have been developed that have a water bath within a vacuum chamber which prevents excessive pressure from being applied to the outer surface of the extrudate as it cools. In order for such vacuum sizing tanks to be effective, the vacuum within the tank must be precisely controlled to provide the correct external pressure on the particular extrudate being processed.
Various attempts have been made to provide a vacuum sizing apparatus with precisely controlled vacuum within the vacuum tank. A recent example of a vacuum sizing tank with electronically controlled vacuum pressure appears in U.S. Pat. No. 5,008,051. The present invention is an improvement over earlier vacuum sizing apparatus in that the present invention provides a very precisely controlled vacuum within the water bath tank.